1. Field of the Invention
The present invention relates generally to sense amplifiers, and more particularly, to methods and systems for dynamically optimizing sense amplifier performance for the current, local temperature of the sense amplifier.
2. Description of the Related Art
Integrated circuits (ICs) typically have thermal gradients across the IC. The thermal gradients are typically caused by different functions being carried out in one portion of the IC than in another portion of the IC because the different functions can lead to variations in power dissipation across the IC. FIG. 1 shows a temperature gradient across a typical microprocessor die 100. The hottest area 102 can have a relatively high operating temperature such as about 105 degrees C. or higher. An adjacent area 104 is slightly cooler at approximately 80 degrees C. An even cooler area 106 is approximately 50 degrees C. The remaining area 108 is approximately 20 degrees C.
The hottest area 102 can include portions of the core processor unit, which performs functions nearly every clock cycle. By comparison, some of the ancillary portions of the microprocessor die 100 such as memory registers and other, non-core functions are operate at cooler temperatures because these ancillary portions may not perform a function every clock cycle or for other reasons described below. As a result, these ancillary portions are typically cooler areas of the microprocessor die 100.
Some portions of the microprocessor die 100 may also have higher device densities than other portions of the microprocessor die 100. The higher density portions, such as the processing core, would typically have a higher temperature than less dense areas 104, 106, 108, because more operations and current flow occurs in higher device density areas. Therefore a temperature gradient can often result between the different areas 102, 104, 106, 108 of the microprocessor die 100.
One of the problems with having the temperature gradient across an IC is that sense amplifiers that are used to read a memory register, or any other type of memory device, are typically designed for the worst-case operating temperature. For example, if a first portion of an IC is an area that operates at about 105 degrees C. and includes about 2–5 percent of the sense amplifiers. A second portion of the IC operates at 20 degrees C., and includes 95–98 percent of the sense amplifiers. Therefore, in the worst-case operating temperature across the IC is the 105 degrees C. and all of the sense amplifiers in the entire IC are typically designed to operate in the range of 105 degrees C. As a result, the performance of 95–98 percent of the sense amplifiers is not optimized for their typical operating conditions.
Sense amplifiers are typically constructed from MOS transistors (NMOS and PMOS). The current flow, Id through a saturated MOS transistor can determined by the following relationship:Id=1/2*u*Cox*W/L*(Vgs−Vt)2  Relationship 1u is the mobility of electrons in NMOS (or holes in PMOS). Cox is the gate electrode capacitance. W/L are the physical dimensions of the device. Vgs is the gate-source voltage applied across the gate-source junction. Vgs is equal to bias voltage as will be described below. Vt is the turn-on, or threshold voltage. By way of example, as temperature goes up, the mobility parameter will shift according to the following relationship:u=uo(300/T)3/2  Relationship 2uo is the mobility of electrons (or holes) @ 300 Kelvin. As T goes up, u goes down. Also as u goes down, physically larger device dimensions (W/L) are required to maintain the same level of current as shown above in Relationship 1. However, when larger devices are used, the relative current must be increased even more due to the additional capacitance resulting from the physically larger devices. Capacitance is typically proportional to physical area of the devices.
As shown in Relationships 1 and 2 above, a sense amplifier designed to operate at 105 degrees C. cannot detect or resolve voltage differences (i.e., switch) as quickly as a sense amplifier that is designed to operate at 20 degrees C. (when operated at 20 degrees C.), with the same current flow, because the larger devices were required in the design targeted for higher temperature operation. A higher current flow is required to match the switching speed. Therefore, the result of having all the sense amplifiers designed for the worst-case operating temperature (i.e., 105 degrees C.), is that the overall performance of all the sense amplifiers is degraded. Further, a sense amplifier designed to operate at 105 degrees C. uses physically larger devices (i.e., have a larger area) than a sense amplifier designed to operate at 20 degrees C. Physically larger devices also have larger node capacitances and also require a greater operating current in the sense amplifier and in the constant current source, than a similar sense amplifier that is designed to operate at 20 degrees C.
The time required for a sense amplifier to switch (i.e., switching time (ΔT)) is defined by the following relationship:ΔV=(I*ΔT)/C  Relationship 3
Where ΔV is the change in voltage across the capacitance C of a node and I is a charging or discharging current across the capacitance C. As shown in Relationship 3 above, the switching time is proportional to the capacitance of the node or device junction in the sense amplifier. Therefore, as the capacitance increases the switching time also increases.
Designing all sense amplifiers to operate at 105 degrees C. when not all sense amplifiers will actually operate at 105 degrees C. will unnecessarily increase the size of the devices required, increase the internal node capacitances leading to increased switching time. Further, the larger device sizes of the sense amplifiers to operate at 105 degrees C. limit the possible locations of the sense amplifiers.
Further, a physically larger device consumes more current to function at a cooler temperature than a smaller device at the same cooler temperature. By way of example, in a typical IC, a sense amplifier designed to operate at 105 degrees C. may consume 200 microampere at 105 degrees C. (the worst case temperature) to switch in time T. The same sense amplifier circuit operated at a lower temperature (e.g., 20 degrees C.) will draw excess current, as shown in Relationship 1 above, and will also switch faster than required under worst case conditions. Therefore, if all sense amplifiers are designed to operate at 105 degrees C. when only about 2 percent are actually operating at 105 degrees, then the remaining 98 percent are consuming excess current which is inefficient from a power perspective in addition.
Another problem specific to some ICs, such as a microprocessor and other processor-type ICs is that depending upon the actual function being performed, the temperature gradients may migrate around the IC. For example, a first portion of the IC may be very hot when performing a first function. Alternatively, when the IC is performing second function, the first portion may be substantially cooler because another portion of the IC is performing the bulk of the second function. Therefore, it is not efficient to design sense amplifiers in a first portion of the IC to be optimized for operating at 20 degrees C. and sense amplifiers in a second portion of the IC to be optimized for operating at 105 degrees C. when the actual operating temperatures of each portion of the IC can vary significantly.
Further still, as device densities have increased, the temperature gradients have similarly increased. By way of example, some current generations of ICs have temperature gradients as much as 50 degrees C. or more across the IC. In one current generation CPU the operating temperature can be 105 degrees C. or hotter in the hotter portions of the CPU and 50 degrees C. or less in the “cooler” portions of the CPU. Designing sense amplifiers throughout the entire CPU to operate at 105 degrees C. is very inefficient use of power and also results in slower switching (i.e., lower speed) sense amplifiers, which degrades overall CPU performance.
Typically the registers and memory locations are spread throughout an IC. One or more sense amplifiers are co-located with each memory cell (or set of memory cells). FIG. 2 is a schematic of a typical sense amplifier 200. The sense amplifier 200 is a typical differential-type amplifier that is capable of resolving small voltage differences and producing a large output voltage. The sense amplifier 200 includes four transistors 202, 204, 206, 208.
The performance of the sense amplifier 200 is controlled by several amplification control parameters such as bias voltage and bias current. Transistor 212 provides a constant current source that is controlled by a bias voltage applied to the gate of transistor 212. As shown transistor 212 is an n-type device so therefore the bias voltage is an Nbias. The bias voltage is typically supplied by an bias voltage source that is local to the particular sense amplifier 200 or a set of sense amplifiers. The bias voltage is a nominal constant voltage from the bias voltage source. The actual bias voltage required is dependant on the type of device. In one typical device, the bias voltage is typically 0.5 VDC. The bias voltage biases transistor 212 at saturation so as to conduct a constant current IBIAS.
Constant current IBIAS is equal to the sum of I1 and I2 flowing across transistors 204 and 208, respectively. While the sum of I1 and I2 is constant, current I1 is greater than I2 if the voltage is applied to the gate of transistor 204 is greater than the voltage applied to the gate of transistor 208. Conversely, current I2 is greater than I1 if the voltage is applied to the gate of transistor 208 is greater than the voltage applied to the gate of transistor 204. An output signal is taken from output terminal 210.
The gain (Av) of the sense amplifier 200 is equal to the product of resistance (RL) of the load transistors 202, 206 and the trans-conductance (gm) of the input transistors 204, 208. The slew rate (i.e., switching time) of the sense amplifier 200 is dependant on the ratio of the IBIAS and the output capacitance (Co). Therefore, a changing in the bias current IBIAS can change the slew rate (i.e., increase or decrease the switching time) and the gain (Av) of the sense amplifier 200. Similarly, a change in the bias voltage can change the bias current IBIAS.
In view of the foregoing, there is a need for a system and method for adjusting the amplification control parameters for each sense amplifier according to the local thermal characteristics of the sense amplifier.